The present invention relates to energy generation systems and, more particularly, to a natural convection boiling-water reactor of the type employing free-surface steam separation. Such reactors typically utilize fission to generate heat. A major objective of the present invention is to provide for enhanced power regulation in a natural convection reactor, for example, to provide load-following ability.
Fission reactors rely on fissioning of fissile atoms such as uranium isotopes (U233, U235) and plutonium isotopes (Pu239, Pu241). Upon absorption of a neutron, a fissile atom can disintegrate, yielding atoms of lower atomic weight and high kinetic energy along with several high-energy neutrons. The kinetic energy of the fission products is quickly dissipated as heat, which is the primary energy product of nuclear reactors. Some of the neutrons released during disintegration can be absorbed by other fissile atoms, causing a chain reaction of disintegration and heat generation. The fissile atoms in nuclear reactors are arranged so that the chain reaction can be self-sustaining.
To facilitate handling, fissile fuel is typically maintained in fuel elements. Typically, these fuel elements have a corrosion-resistant cladding. The fuel elements can be grouped together at fixed distances from each other in a fuel bundle. A sufficient number of these fuel bundles are combined to form a reactor core capable of a self-sustaining chain reaction. Neutron-absorbing control rods are inserted into the core to control the reactivity of the core. The reactivity of the core can be adjusted by incremental insertions and withdrawals of the control rod.
Reactors can be classified according to the method used to transfer fission-generated heat from the reactor core. In boiling-water reactors, water is converted to steam as it flows through the core. The steam can be conveyed from the reactor vessel enclosing the core to a turbine. The steam drives the turbine which, in turn, drives a generator to produce electricity. Forced-circulation boiling-water reactors utilize pumps to force water to circulate up through the reactor core and along a return path to the base of the core. "Carryover", i.e., water carried with the separated steam is removed by dryers, and the steam escapes from the vessel into a conduit to the turbine. The separated water returns to the forced-circulation system.
Natural-convection boiling-water reactors (NCBWRs) limit complexity by implementing coolant circulation without a circulation pumping subsystem. A chimney supports a steam column above the core, and this steam column serves as a driving head for the water circulation. Water circulating up through the core and the chimney is at least partially converted to steam which forms a relatively low pressure head above the core. Water recirculates down a downcomer annulus between the reactor vessel and the chimney and core. The water in the downcomer is denser than the steam and water mixture in the core and chimney region. The difference in density forces water up through the core and chimney and down through the downcomer.
Power regulation has been problematic in NCBWRs. As with other reactor types, the power output by a NCBWR can be varied by moving the control rods. However, moving control rods fatigues the cladding on the fuel elements so that this method of power regulation must be used sparingly. Control rods more effectively dampen fission in nearby fuel than in more distant fuel. A control rod inserted from below dampens fission more effectively in fuel located below the top of the control rod than above this level. Accordingly, the fuel elements are hotter above the control rod level than below the control level. Concomitantly, there is a vertical thermal gradient along the fuel element cladding which is steepest near the control rod top level. This thermal gradient is associated with substantially mechanical stress in the cladding. Each time a control rod is moved, the steep thermal gradients move along the cladding, which is thereby fatigued. Thus, while moving control rods is useful for infrequent gross power adjustments, it is not a viable approach to ongoing real-time power regulation.
In forced-circulation boiling-water reactors, power output can be regulated by controlling the pumping speed of the circulation system. Water is an effective moderator, i.e., it slows high energy neutrons so as to increase their likelihood of absorption. Steam is not an effective moderator. Faster pumping increases the velocity of water flowing through the core, and thus the mean height of the water in the column before it is vaporized. Thus, faster pumping results in more water and less steam in the core. Slower pumping causes vaporization at a lower point. Thus, greater pumping speed means more moderation and thus a stronger chain reaction, more heat generation, and a greater power output.
The effect of changing pumping speed on reactivity is much less than that obtainable moving control rods. Thus, changing pumping speed is useful for modulating power output about a gross level set using the control rods. The weaker effect of the pumping speed implies smaller effects on thermal gradients and stresses on the cladding. Moreover, the transition from water to steam as a function of core height is gradual; thus, changing pumping speed does not cause a singularly steep thermal gradient to move along the cladding. Therefore, modulating power output by regulating pumping speed does not significantly fatigue fuel-element claddings. Thus, pumping speed can be used on an ongoing basis to control power output in a forced circulation boiling-water reactor.
By definition, NCBWRs lack this recirculation pump and therefore power output cannot be controlled as a function of pumping speed. Some regulation can be provided by adjusting a valve on the steam line from the vessel to the turbine. Closing the valve reduces the steam reaching the turbine, reducing the drive on the turbine and, in turn, on the generator. However, closing this valve also causes a pressure buildup in the vessel. This forces more steam through the closed valve, offsetting the original power reduction. The net effect is too limited to provide useful load-following ability for a NCBWR. Another disadvantage is that the increase in vessel pressure is a safety concern.
The lack of load-following ability in NCBWRs has been circumvented in practice since existing designs have been part of larger networks of nuclear and non-nuclear electric power stations. Since the power output of fossil fuel power stations is easily regulated, the nuclear power station can be operated continuously at full power. The total power is adjusted at the fossil fuel stations. However, more powerful NCBWRs are being developed whose full generating capacity will not necessarily be absorbed at all times by the consumers. What is needed is an improved means for regulating power output which safely provides a substantial load-following power range without significantly fatiguing fuel elements.